Hi I'm jackie,
Recently I started talking to my half brother. I am wondering if my father (his father) would have to pay child support for my brother even though he is 27 and his birth certificate says his step-dads name on it.

Whoever the legal father is is the man liable for child support. The man on the birth certificate is the legal father. At this point, it doesn't matter... child support is paid to a PARENT for the care of a minor child. As he is no longer a minor, a new support order would not be issued. If there was an previously existing support order naming your father, he would be liable for any arrears owed under that order.

Yes, a grown man does not get child support, if he is 27, the time for child support is long past.
His mom would have gotten that over 10 years ago